That's the technique Michael! Works great. Why didn't I think of doing it that way?

This also works for that rudder area. I've extended the curves down and trimmed. With care I might be able to extend it right to the stern without need for any networking at all, for a one piece hull.

I've included a quick render and the result looks near perfect.

The key points I think are to extend and trim and to keep the same number of curve segments.

Thanks Michael and thanks Danny and Burr for your input.